Feasibility of In-Vivo Cardiac HIFU Ablation

The potential for cardiac applications of HIFU remains largely unexplored. In order to create reproducible lesions in a beating heart, it is necessary to maintain focusing at a certain position within moving myocardial tissue. One technique is to use multiple short HIFU exposures (0.2 s) and to synchronize them with an EKG signal and respiration. In order to investigate the interaction of HIFU exposures and cardiac tissues, a series of in-vitro experiments was conducted. The left ventricular free wall (LVFW) of calf hearts were cut into 4-cm cubes, degassed in phosphate buffer saline (PBS), and heated to 37C. Several transducers were employed. Most experiments used a 33-mm diameter spherical-cap transducer with focal length of 35 mm, operated at a frequency of 5.075 MHz and a focused intensity of 13 kW/cm2 (in-situ spatial average over the half-power points of the focused beam). The transducer was coupled to the LVFW using degassed PBS. First, the effects of pericardial fat, focal depth, and temperature on lesion size were individually evaluated. We compared the effect of different time pulse duration (0.2 s and 0.3 s) and number of applied pulses. Dimensions of all lesions were measured by visual examination of the fresh, unstained tissue. Histopathological examination of the lesions was also performed.